Point sources

The processing sequence for performing a spectral fit on a point source (once the data files have been converted) is as follows.

1. Create an image of the whole field using XSORT

   ICL> XSORT  SRCFILE=CYGX1_IMAGE
   

2. Display the image using the 'I' routines.

   ICL> ILOAD  CYGX1_IMAGE
   ICL> IDISPLAY
   

3. Choose source and background box centres and sizes

   ICL> XRTBCKBOX
   \begin{quote}\begin{verbatim}
    
   This will allow you to use a cursor on the image to select
   the position and radius of a source and background box.
   It then sorts events from within these boxes into a source
   and background spectrum.
    
   \item Background subtract the source file
    
   \begin{quote}\begin{verbatim}
   ICL> XRTSUB
   \begin{quote}\begin{verbatim}
    
   This corrects the background spectrum to that which would have
   been seen had this background been at the position of the source
   box. It then subtracts it from the source spectrum.
    
   NB: the correction of the background spectrum to the source
   position is a vignetting correction which is performed on a
   pulse height channel basis. In reality vignetting corrections
   are only calibrated for photon energies. So a small third order
   error is introduced by assuming that we know the "energy" of
   each corrected pulse height bin.
    
   \item Correct the subtracted spectrum
    
   \begin{quote}\begin{verbatim}
   ICL> XRTCORR
   

   This will correct the spectrum for the energy independent effects, dead time and wires. In addition it produces an array of energy dependent correction factors within the datafile. These are used as the effective area at each energy for this datafile.
4. Produce a detector response matrix

   ICL> XRTRESP
   

   This multiplies the detector matrix file with the energy corrections array found in the corrected datafile. The resultant energy response structure is stored in the datafile.

5. Set up a spectral model and fit the data

   ICL> SMODEL
   ICL> SFIT